doc: abort_source: fix dangling \ref
doc: abort_source: fix dangling \ref

Resolves the following doxygen warning:
include/seastar/core/abort_source.hh:130: warning: unable to resolve reference to 'if' for \ref command

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,dreamsxin/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,cloudius-systems/seastar